The first warm Saturday of spring, I hauled the mower out of the shed, gave the cord a yank, and got back nothing but a wheeze. New plug, cleaned the air filter, checked the kill switch — still nothing. The fault wasn’t anything I could unbolt. It was the half-tank of petrol that had sat there since autumn, quietly turning to varnish while the mower waited out the winter. Stale fuel is the most common reason a small engine won’t start after a lay-up, and the most overlooked.

Petrol does not last forever. It is a volatile, reactive blend that starts changing the day it leaves the refinery, and in a vented mower or chainsaw tank it can be well past its best in a matter of months. The good news is you can catch it in about three minutes, with your nose and a clear jar, before you waste a morning chasing a fault that isn’t there.

Best practical rule: if a petrol engine has sat unused for more than a few months and won’t start, suspect the fuel first — before the spark plug, before the carburettor, before anything you can take apart. Old petrol is far cheaper to rule out than a stripped carburettor.

Here is how long petrol really lasts, how to tell at a glance whether yours has turned, what it does to an engine once it has, and how to stop it happening again. If the mower is due an oil change while it’s out anyway, that’s covered in what oil a lawn mower takes.

⌛How Long Petrol Actually Lasts

Petrol has no stamped expiry date, but there are realistic windows. Sealed in a cool, full, airtight container, ethanol-free petrol stays usable for roughly six to twelve months. Left in a half-empty, vented small-engine tank through a damp winter, the same fuel can be noticeably stale in one to three months — sometimes less in a hot shed.

Here is the good news for us: most petrol sold in New Zealand is ethanol-free, which matters because ethanol is what sends fuel off fastest — it pulls water straight out of the air. Some ethanol blends (E10) are sold here, so it pays to read the pump if you’re unsure, but a tank of ordinary 91 from most stations won’t contain any. In Australia and the United States, where E10 is the norm, stale-fuel trouble arrives sooner.

Two things shorten every figure in that table: air and warmth. A vented tank breathes moist air in and out with each temperature swing, and heat speeds up the chemistry. A sealed steel or approved plastic can in a cool cupboard is a different world from a translucent jerry can baking in the sun on the deck.

👃The Three-Minute Test: Smell, Look, and a Clear Jar

You don’t need a lab. Three quick checks tell you almost everything, and together they take about as long as boiling the jug.

Smell it

Fresh petrol smells sharp and solventy — the smell you know from the forecourt. Stale petrol smells dull, sour, or varnishy, closer to old paint thinner or turps, sometimes faintly sweet and rancid. If a sniff makes you wrinkle your nose in a way fresh fuel never does, trust that. Your nose is a better stale-fuel detector than most gauges.

Look at the colour

Decant a little into a clean, clear glass jar and hold it up to the light. Fresh petrol is clear and almost colourless, sometimes with a faint straw or blue-green tint from dye. As it ages it darkens — pale amber first, then orange, then a tea-brown, varnish-like colour in the worst cases. The darker and murkier it looks next to fresh fuel, the further gone it is.

Check for water and a second layer

While it’s in the jar, look for cloudiness or a distinct layer sitting at the bottom. That bottom layer is water — and in ethanol fuel, the ethanol bonded to it — separated out of the petrol where it will not burn. If you can see it, the fuel isn’t merely stale, it’s contaminated, and no amount of fresh petrol on top will fix what’s sitting underneath.

⚗What’s Actually Happening: Why Petrol Goes Off

Three things go wrong at once, and it helps to know which one is biting you.

The light ends evaporate

Petrol is a blend of many hydrocarbons, some far more volatile than others. The lightest, most flammable fractions — the ones that make a cold engine catch — are exactly the ones that boil off first, especially from a vented tank. Lose them and what remains is harder to ignite. That is why stale fuel so often cranks but won’t fire, or only catches on full choke.

It oxidises into gums

Petrol reacts slowly with oxygen. Boiled right down — and this is a simplification, because petrol is hundreds of compounds rather than one tidy molecule — the first step looks like this:

R-H + O₂ → R-O-O-H

In plain English: a hydrocarbon in the fuel grabs oxygen and turns into an unstable peroxide. Those peroxides react on and link together into heavier, sticky gums and varnishes, throwing off small amounts of acid as they go. The gums are the brown lacquer that blocks carburettors; the acids are what corrode the metal. This is the damage you can’t reverse — once the fuel has gummed, it has gummed.

Ethanol pulls in water

If the fuel does contain ethanol, there’s a fourth problem. Ethanol is hygroscopic — it attracts and holds water out of the air. Past a certain point the water-and-ethanol mix can no longer stay dissolved in the petrol and drops out as a separate layer at the bottom of the tank. That’s phase separation, and the bottom layer is corrosive, won’t combust, and drags some of the fuel’s octane down with it. It’s the main reason ethanol blends go off so much faster than our ethanol-free petrol.

🔧What Stale Fuel Does to a Small Engine

Small engines suffer worse than cars. Their tanks are vented, their carburettor jets are tiny enough to block with a speck of gum, and they sit idle for months at a stretch. The symptoms follow a familiar pattern:

• It cranks but won’t start. The volatile fractions that make cold starting easy have evaporated, so there’s nothing light enough left to catch.
• It only runs on choke, or dies the moment you back the choke off. Gum has narrowed the tiny idle and pilot jets, so the engine can’t draw a clean mixture without the choke compensating.
• It starts, then stalls under load. Partial blockages let it idle but starve it the moment you ask it to actually cut grass.
• The carburettor bowl is full of crud. Drain it and you’ll often find dark varnish, a sour smell, or green-white corrosion — the gums and acids made solid.
• It floods easily. Gummed needle valves and sticky floats let fuel through when they shouldn’t. If yours is drowned right now, here’s starting a flooded weedeater.

If you’ve confirmed the fuel is fresh and it still won’t go, the next cheap thing to rule out is the plug — setting the spark plug gap walks through it.

🧰What to Do with Stale Petrol (and the Engine It’s In)

Once you’ve found it, deal with the fuel and the engine as two separate jobs.

1. Decide: dilute or dispose. If the fuel is only mildly stale, ethanol-free, and shows no water layer, you can dilute it heavily — a litre or two tipped into a near-full car tank does no harm. If it’s dark, sour, cloudy, or has a layer at the bottom, don’t burn it in anything you care about. Dispose of it.
2. Empty the tank. Siphon or drain the old fuel out of the engine completely, outside, well away from sparks — and mind the vapour.
3. Drain the carburettor bowl. This is the step people skip. Fresh fuel in the tank does nothing about gum already sitting in the float bowl and jets. Most carburettors have a drain screw at the base; open it and let the bowl empty.
4. Refill with fresh petrol, ideally with a carburettor or injector cleaner added to help dissolve light gum as the engine runs.
5. If it still won’t run cleanly, clean the carburettor. Light gum often clears with cleaner and a few minutes’ running. Heavier deposits mean pulling the carburettor, soaking it, and clearing each jet with carburettor cleaner and a fine wire.
6. Check the plug last. A fouled or wrongly gapped plug can carry over from before the lay-up — again, setting the spark plug gap covers it.

🛡️Stop It Going Off in the First Place

This is where the real win is. Stale fuel is far easier to prevent than to cure, and the prevention costs minutes.

The honest correction: fuel stabiliser protects fresh petrol — it does not revive stale petrol. Once the light ends have evaporated and the gums have formed, no additive brings them back. Stabiliser is insurance you buy in advance, not a repair you make afterwards.

• Buy small. For mowers and trimmers, buy only what you’ll burn in about a month. A cheap small can beats a bargain 20 litres that slowly turns to varnish.
• Store it sealed and cool. An airtight, approved container in a cool, shaded spot, kept fairly full — less air space means less oxygen and less moisture getting at the fuel.
• Stabilise it for storage. Heading into winter, add a fuel stabiliser [AMAZON LINK: fuel stabiliser, e.g. STA-BIL] to fresh fuel, then run the engine a few minutes so treated fuel reaches the carburettor.
• Or run it dry. The simplest off-season trick: run the engine until it stops, then store it empty. No fuel in the carburettor means nothing to gum up. Some makers prefer storage with stabilised fuel instead — check your manual.

Treat the start of winter as the moment to handle this, the same way what oil a lawn mower takes treats the off-season oil change. A few minutes in autumn saves a no-start the first warm weekend in spring.

❓Frequently Asked Questions

How long does petrol last in a mower or chainsaw?

In a vented small-engine tank, often just one to three months before it starts causing hard starting — and less in a hot shed. Sealed in a cool can, ethanol-free petrol keeps for six to twelve months.

Can I just top up stale petrol with fresh?

Topping up dilutes the problem but doesn’t fix gum already in the carburettor. For a mildly stale tank, fresh fuel plus a carburettor cleaner can be enough. If it’s dark, sour, or has a water layer, drain it instead.

Does premium 95 or 98 last longer than 91?

Not meaningfully. Octane rating is about resistance to knock, not shelf life. Storage conditions and whether there’s ethanol in the blend matter far more than the grade you bought.

Is New Zealand petrol ethanol-free?

Most pump petrol here is ethanol-free, which helps it keep longer than the E10 common overseas. Some ethanol blends are sold, so check the pump sticker if it matters for an older engine or long storage.

Will fuel stabiliser fix petrol that’s already stale?

No. Stabiliser slows the ageing of fresh fuel; it can’t reverse oxidation or bring back the light ends that have evaporated. Add it before storage, not after the fuel has turned.

How do I dispose of old petrol?

Keep it in an approved container and take it to a transfer station or hazardous-waste collection — many councils accept it free. Never pour it down a drain or onto the ground.

The takeaway is simple: petrol is perishable, small engines are unforgiving about it, and thirty seconds with your nose and a clear jar will save you stripping a carburettor. Buy it fresh, store it sealed and cool, and deal with it heading into winter — not in spring, when the lawn’s already away on you.

This pairs with what oil a lawn mower takes for the rest of your off-season service, starting a flooded weedeater for when an engine is drowned rather than starved, and setting the spark plug gap for the other half of most no-start mysteries. For more shed-tested fixes, dig into the tips and tricks section.